In Vitro Transformation of Inorganic Iodine to Protein-Bound Iodine in Milk by Formaldehyde

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Abstract
Addition of formaldehyde (HCHO) to raw skimmilk causes transformation of I131 to protein-bound iodine-131 (PB131I). Xanthine oxidase (XO) in milk oxidizes HCHO to H2O2 in the presence of O2, and H2O0 oxidizes 131I- to 131 I 2 which binds to protein. Factors that affect XO activity, like temperature and [HCHO], affect the amount of PBI131 formed. The optimum temperature for maximum PBI131 formation was 65 C. ArrheIllus. plot of 1/T versus log K*, the rate constant, gave activation energy of 20.92 K cal mole-1 for the over-all reaction. Data relating 1/s and 1/v gave Vmax of 93.4.[plus or minus]2.0% and Km which varied with the rate of PBIl31 formation, indicating competitive inhibition type of reaction. The amount of PBI131 formed, HCHO and XO activity were linearly correlated. The pH of milk (6.0 - 7.0) affected v but not the Vmax-The optimum [I-] oxidized at 60 C, and for 1.31 x 10-2 M HCHO, was 2 [mu][image]/100 ml of milk. Analysis by chromatography showed that 65 -68% of the PBI131 was in the monoiodotyrosine, 6-77% in the di-iodotyrosine, 2.9 - 3.5% in the thyroxine and 8% in the I- form. Nonprotein constituents of milk inhibiting the XO-catalyzed reaction were removable from milk by dialysis or treatment with anion exchange resin or with activated C.